The general process for modifying iron, and in particular producing nodular cast iron, (i.e., cast iron comprising nodular or spheroidal graphitic inclusions) comprises in its broadest aspect supplying to molten grey iron a relatively minor amount of magnesium (based on the weight of the cast iron to be treated). Such magnesium additions preferentially lowers the sulfur and oxygen content of molten cast iron compositions, and, if sufficient magnesium is added, such treatment has the effect of producing spheroidal graphite rather than a flake graphite form.
Considerable problems have been associated with the introduction of elemental magnesium to a bath or exposed stream of molten iron. Ladle additions of magnesium to a molten iron bath have been largely avoided because the comparatively low boiling point of magnesium and its high degree of reactivity with oxygen and low density (relative to the density of the molten cast iron) causes substantial and expensive magnesium losses resulting from flash off at the surface of the molten bath. The loss is usually indicated by a violent pyrotechnic display and is accompanied by a violent reaction causing splashing of molten iron; this latter factor, along with the pyrotechnic display, constitute a serious threat to the welfare of personnel and equipment, especially in commericial operations wherein the amount of iron to be treated and the amount of magnesium metal required is generally great.
Efforts to reduce pyrotechnics and splashing have usually comprised adding the nodularizing agent to an enclosed treating ladle or enclosed reservoir stationed in the mold and through which the metal must ultimately flow. A modification to the ladle addition approach has used a tubular device to introduce solid addition agents, such as magnesium, below the surface of the molten metal; the magnesium is added in the form of a fine grain suspension in a gaseous carrier. Similar to this approach is the sub-surface injection of a mixture of powdered carbon and elemental magnesium, or the use of a tiltable reaction ladle with magnesium stored in one region thereof and caused to react under a certain vapor pressure. A commercial ladle approach, is the dropping of powdered additives of magnesium through a chute that enters a conical cavity in a stream of molten iron flowing through an aperture in the bottom of a storage chamber for molten metal; this is commonly referred to as the T-nock process. There is a vast number of other published arrangements for introducing magnesium during the pouring of molten iron into a ladle or while it is in the ladle.
In all of the above enclosed pouring ladle approaches, the results are unsatisfactory because of essentially three problems, the most important of which is that the metal must be superheated to accommodate the considerable loss in heat from reladling and pouring. The superheat destroys growth sites and thus requires post inoculation to improve the distribution of the graphite nodules; inoculation has never achieved totally satisfactory homogeneity and magnesium recovery is relatively low leading to high costs.
The other two problems comprise dross build-up in the pouring vessel and the fading of the reacted magnesium before solidification. Dross on the ladle refractories create magnesium reaction products (sulfides, oxides); this can lead to excessive pouring unit downtime as a result of inductor channel clogging, loss of vessel volume, and pouring orifice restrictions. Magnesium and post inoculant fade are time dependent phenomenon. In general, the iron must be poured within 15 minutes of the time of treatment. If this cannot be done and if corrective actions are not taken, low nodularity of carbidic castings are likely to result.
Thus the prior art has turned to treating the molten iron after it leaves the mechanical pouring unit or ladle. One general approach to this post treatment is that which treats the molten metal as it flows through the casting mold or just prior to its entrance into the mold cavity. A notable example of stream treatment employs a reaction chamber embedded in the sand mold, thus forming a part of the runner system. A charge of magnesium bearing material is added to the reaction chamber in advance of pouring. Nodularization is accomplished by the reaction of this magnesium bearing material with the molten metal flowing through the reaction chamber. Several disadvantages are associated with this process including increased casting inspection, the ratio of the poured weight of metal to the cleaned weight of metal increases, there must be closer metal-lurgical control, an investment in unique runner and gating systems, and usually a closely sized magnesium ferrosilicon alloy is required since the molten metal has a difficult time in flowing around each magnesium particle. As to the increased casting inspection, this becomes a significant disadvantage. Each mold is treated individually. The conventional method of checking each treated quantity of metal for nodularizing content and for chill is impractical. A fail safe method of adding the magnesium alloy and of checking the produced castings has yet to be developed to make this approach successful.
Earlier attempts at stream treatment used a filter element placed at the mouth of this mold gating system; the filter had a predetermined porous magnesium matrix through which the molten iron was poured. Alternatively, a consumable pouring sprue containing sponge iron impregnated with magnesium, both of which were reacted at a predetermined rate of consumption. In still another approach, an exposed stream was poured into a mold and an exposed stream of magnesium additive was projected against the stream for mixing and chemical reaction. These earlier attempts at stream treatment were, of course, unsatisfactory because they did not provide a controlled rate of solution; this is a function of alloy form and composition, treatment temperature, system heat, type and time of exposure to iron (the solvent) and oxygen available.
Whether the commercial practice has been stream treatment or ladle treatment, it has been carried out in batches; typically, up to several tons of molten iron is nodularized in a treating or holding ladle, then reladled into several pouring ladles, and then finally poured into a mold with post inoculating agents added to the pouring stream during transfer from treating ladle to the pouring ladle.